1. Technical Field
The present invention relates to a turbo compound engine provided with a power turbine for recovering the exhaust gas energy and returning the recovered energy to the crankshaft of the engine. More particularly, the present invention relates to a turbo compound engine system which produces braking effort by rotating the power turbine in a reverse sense by the crankshaft during engine braking.
2. Background Art
The Assignee of the present invention has proposed a "Turbo Compound Engine" (Japanese Patent Application No. 61-228107), which is a system for obtaining braking effort by rotating a power turbine in a reverse sense with the power turbine being provided in the exhaust gas line of the engine.
In this proposal, as depicted in FIG. 6, a power turbine 51 for recovering the exhaust gas energy is provided in the exhaust gas line 53, a fluid passage 57 which bypasses the power turbine 51 is connected to the exhaust gas line 53, and a fluid passage switching means 55 is provided to the fluid passage 57. The fluid passage switching means 55 is constructed such that the exhaust gas line 53a parallel to the fluid passage 57 may be closed when the exhaust brake is applied and rotation power is transmitted from the crankshaft 59 to the power turbine 51 while opening the fluid passage 57.
The power turbine has to be driven at a rotational speed between 80,000 and 100,000 rpm in order to be effectively utilized. Therefore, when the power turbine 51 is reversed in its rotational direction, considerable load is imposed on a power transmission system 61 between the crankshaft 59 and the power turbine 51.
As indicated by braking effort characteristic curve I in FIG. 4, if the power turbine functions as a compressor as the rotation of the power turbine is reversed from normal direction to reverse direction, the load against the crankshaft sharply increases, which is called overshoot, just after the rotation of the power turbine reaches zero, i.e., just after the power turbine starts rotating in a reverse sense. This is because the power turbine performs both compressor work and air mixing work when it is reversed. To overcome this shortcoming when the turbo compound engine of the above proposal is employed, the power transmission system between the crankshaft and the power turbine has to possess strength enough to counterbalance the overshoot or the power turbine has to be rotated slowly enough not to damage the power transmission system. Either case is undesirable in terms of total braking effort and cost.